1. Field of the Invention
This invention relates to a malfunction detector for an acceleration sensor provided in an antilock control system of a vehicle.
2. Description of the Related Art
The deceleration of a vehicle while its wheels are in a locked state represents the frictional coefficient of the road surface (hereinafter denoted .mu.), provided the road surface is uniform. Thus, by providing an antilock control system with an acceleration sensor, it becomes possible to judge whether the vehicle is travelling on a road surface having a high-.mu. value or one having a low-.mu. value. This in turn makes it possible to change over the control mode in such a manner so as to increase the locking detecting sensitivity or to increase the sensitivity for detecting the recovery from a locked state while the vehicle is on a low-.mu. road, thus reducing the brake pressure to a greater degree, and to lower such a detecting sensitivity while on a high-.mu. road, thus reducing the brake pressure to a lesser degree.
In addition to the above method, various proposals have been made to change the processing level of antilock control, depending upon the ON and OFF positions of an acceleration sensor. For example, it was proposed to re-increase the brake pressure after pressure reduction while on a high-.mu. road and slowly while on a low-.mu. road, or to control the rate of change of the estimated vehicle speed.
Such acceleration sensors come in various structures. FIG. 2 shows on example which includes a switch body 1 and a glass tube 2 fixedly mounted in the switch body 1 at an angle .beta. with respect to the mounting surface M. If a deceleration larger than tan .beta. acts while the vehicle is moving in the direction of the depicted arrow (i.e. parallel to the mounting surface), mercury 3 in the glass tube 2 moves toward electrodes 4, electrically connecting the electrodes 4 together. While the electrodes 4 are electrically connected, a deceleration which is larger than a predetermined level is present. If the electrodes 4 are not so connected, then no such deceleration is present.
In contrast, electrodes may be provided at the lower end of the glass tube 2 so that an electrical connection will be cut off while a deceleration which is larger than a predetermined level is present. Another type of acceleration sensor has rolling elements adapted to roll when subjected to deceleration to actuate a limit switch.
If such acceleration sensors should fail, it becomes impossible to change the control level of the antilock control system. This poses a problem in that the braking distance increases.
To prevent this problem, it was proposed to provide a plurality of acceleration sensors and to carry out control for a high-.mu. road according to one of two methods: (1) if one of the acceleration sensors indicates a high deceleration, or (2) only if all of the acceleration sensors show a high deceleration.
However, the first method has a drawback in that if the mass in one of the acceleration sensors should get stuck at the high deceleration side, the antilock control performance on a low-.mu. road will drop because the control mode is fixed to that for a high-.mu. road.
Further, the second method has a drawback in that if the mass in any one of the sensors should get stuck at the low deceleration side, the braking distance tends to increase while on a high-.mu. road, because the control mode is fixed to one for a low-.mu. road.
Further, even if no mass becomes stuck within the sensors, if the sensitivity of the acceleration sensors should shift for some reason, the same problems as described above will result.
It is an ordinary practice to provide an antilock device with a plurality of acceleration sensors, if provided at all, for safety's sake. In one conventional method for testing whether or not these acceleration sensors are working normally, it is checked whether the acceleration sensor outputs have changed over after inclining the vehicle by an angle at which the outputs are supposed to change over.
However, this method suffers a drawback in that it requires a plurality of audio-visual means for indicating the output states of the respective acceleration sensors.